Projection optical system, magnification projection optical system, magnification projection apparatus, and image projection apparatus

ABSTRACT

A projection optical system for use in an image projection apparatus illuminating a lightbulb forming an image in accordance with a modulating signal with illumination light from a light source is disclosed. The projection optical system includes first and second optical systems arranged along an optical path defining an upstream-downstream direction in the order described from upstream to downstream on the downstream side of the lightbulb. The first optical system includes at least one dioptric system and has positive power. The second optical system includes at least one reflecting surface having power and has positive power. The image formed by the lightbulb is formed as an intermediate image in the optical path, and the intermediate image is magnified and projected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a projection optical system, amagnification projection optical system, a magnification projectionapparatus, and an image projection apparatus.

2. Description of the Related Art

Liquid crystal projectors are widely known as image projectionapparatuses. In these days, the liquid crystal projectors have employedliquid crystal panels with higher resolution, have enjoyed improvedbrightness due to higher efficiencies of light source lamps, and havebecome less expensive.

Further, small, light-weight image projection apparatuses using a DMD(Digital Micromirror Device) have been widely used not only in officesand schools but also at home. Particularly, the portability offront-type projectors has been improved so that they are also used insmall meetings.

It is desired that projectors, or image projection apparatuses, be ableto project a large-screen image. That is, it is desired to enlarge aprojection screen. Further, it is also desired that a projection spacerequired outside a projector be reduced as much as possible.

In order to reduce the outside projection space while enlarging theprojection screen, it is desirable to incorporate the light path of animaging light beam forming a projected image as much as possible in theprojector (image projection apparatus). The following patent-relateddocuments disclose image projection apparatuses employing suchtechnique.

Japanese Laid-Open Patent Application No. 2002-40326 (Prior Art 1)discloses an image projection apparatus including first through fourthreflecting mirrors to obtain a wider angle of view while preventing animaging optical system from becoming larger in size. The firstreflecting mirror has a concave shape and each of the second throughfourth reflecting mirrors has a convex shape. The imaging optical systemis composed of the first through fourth reflecting mirrors. At least oneof the first through fourth reflecting mirrors has a free-form surfaceso as to ensure projection performance.

Japanese Laid-Open Patent Application No. 2002-174853 (Prior Art 2)discloses an image projection apparatus that is a surface projectiontype display whose projection distance to a screen is reduced. Thesurface projection type display includes an imaging optical systemcomposed of a pair of a concave mirror and a convex mirror havingdivergence action and a projection lens.

Japanese Examined Patent Application Publication No. 6-91641 (Prior Art3) discloses an image projection apparatus that is a video projector.The first mirror surface of the imaging optical system of the videoprojector has a convex shape so as to reduce the thickness of the videoprojector.

The image projection methods of Prior Art 1 and 3 perform imageformation using only one or more reflecting mirrors to magnify andproject a single lightbulb image on a screen. Therefore, these imageprojection methods enjoy the merit of no generation of chromaticaberration in principle. In the case of displaying images of red, green,and blue separately, using three lightbulbs instead of a singlelightbulb, and combining the separate images on a screen, theintervention of a color combining part such as a cross prism or aPhilips prism is necessary, thus resulting in the generation ofchromatic aberration at the time of combining colors. However, theimaging optical system composed of only reflecting surfaces cannotcorrect chromatic aberration.

According to an image projection apparatus disclosed in JapaneseLaid-Open Patent Application No. 2001-264627 (Prior Art 4), each lightbeam from an image display panel is guided to a screen successively bythe positive-power imaging lens system and the reflection optical systemincluding a negative-power curved surface mirror of a magnificationprojection optical system to be focused onto the screen.

The screen and the imaging lens system are set so that the height of thescreen is offset from the height of the imaging lens system. The lightbeams are reflected back from the mirror to be guided to the screen.Therefore, the optical path length of the imaging light beam differsbetween the upper side and the lower side of the center part of themagnified projected image on the screen (corresponding to the centerpart of the image display panel), thus resulting in the generation of aso-called “trapezoidal distortion.”

The trapezoidal distortion is correctable by “keystone correction,”which, however, is likely to degrade the quality of the magnified imageon the screen.

It is well known to provide a convex mirror between the imaging lenssystem and the screen so that the convex mirror is decentered withrespect to the optical axis of the imaging lens system in order toreduce the trapezoidal distortion. In the case of providing the convexmirror with decentration, the convex mirror is provided on the imaginglens side of the screen-side focus position of the imaging lens systemso as to extend the focus position of a projection lens by the negativerefracting power of the convex mirror.

In order to realize a low-profile, large-screen magnification projectionapparatus with the above-described configuration, the negative power ofthe convex mirror may be increased to widen an angle of view. This,however, requires stricter shape accuracy and stricter assemblytolerance of the convex mirror, and also increases distortion.

Distortion may be reduced by decreasing the refractive power of theconvex mirror by increasing the distance between the imaging lens systemand the convex mirror. As the distance between the imaging lens systemand the convex mirror increases, however, the convex mirror becomeslarger in size to increase its cost. Further, the magnificationprojection apparatus is also likely to become larger in size.

According to Japanese Laid-Open Patent Application No. 2002-296503, amagnification projection optical system is composed of only reflectingmirrors. In the case of thus obtaining desired optical performancewithout using a lens optical system, it is necessary to set the surfaceaccuracy and position accuracy of each reflecting surface to extremelyhigh values, thus requiring stricter assembly accuracy of themagnification projection optical system.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea projection optical system, a magnification projection optical system,a magnification projection apparatus, and an image projection apparatusin which the above-described disadvantages are eliminated.

A more specific object of the present invention is to provide aprojection optical system, a magnification projection optical system,and a magnification projection apparatus that can increase a projectionscreen in size, employ an imaging optical system including a reflectingsurface to reduce a projection space outside a projection apparatus, andat the same time, correct chromatic aberration.

Another more specific object of the present invention is to provide animage projection apparatus including such a projection optical system.

Yet another more specific object of the present invention is to reducethe thickness of an image projection apparatus and perform projection ona large screen without distortion.

One or more of the above-described objects of the present invention areachieved by a projection optical system for use in an image projectionapparatus illuminating a lightbulb forming an image in accordance with amodulating signal with illumination light from a light source, theprojection optical system including first and second optical systemsarranged along an optical path defining an upstream-downstream directionin an order described from upstream to downstream on a downstream sideof the lightbulb, wherein the first optical system includes at least onedioptric system and has positive power; the second optical systemincludes at least one reflecting surface having power and has positivepower; and the image formed by the lightbulb is formed as anintermediate image in the optical path, and the intermediate image ismagnified and projected.

The above-described projection optical system includes a first opticalsystem and a second optical system. The image formed by the lightbulb isformed as an intermediate image in the optical path of the first andsecond optical systems, and the intermediate image is further magnifiedand projected. Accordingly, high projection magnification can berealized. Since the first optical system includes a dioptric system,chromatic aberration is correctable using the chromatic dispersioncharacteristic even in the case of employing a color composite prism.Further, since the optical path of imaging light beams is folded back bya reflecting surface of the second optical system, the projectionoptical system can be made compact.

One or more of the above objects of the present invention are alsoachieved by an image projection apparatus that illuminates a lightbulbforming an image in accordance with a modulating signal withillumination light from a light source, the image projection apparatusincluding: a projection optical system, the projection optical systemincluding first and second optical systems arranged along an opticalpath defining an upstream-downstream direction in an order describedfrom upstream to downstream on a downstream side of the lightbulb,wherein the first optical system includes at least one dioptric systemand has positive power; the second optical system includes at least onereflecting surface having power and has positive power; and the imageformed by the lightbulb is formed as an intermediate image in theoptical path, and the intermediate image is magnified and projected bythe projection optical system.

The above-described image projection apparatus can be made compact witha long optical path of image light beams being provided in a space inthe apparatus. As a result, a large-size image can be projected anddisplayed with a reduced projection space outside the apparatus.

One or more of the above objects of the present invention are alsoachieved by a magnification projection optical system that guides alight beam from an image display panel to a screen in anupstream-downstream direction, projects the light beam from a directioninclined to a normal of the screen, and forms on the screen a magnifiedversion of an image displayed on the image display panel, themagnification projection optical system including: a reflection opticalsystem; and a transmission optical system, wherein the reflectionoptical system includes a plurality of reflecting surfaces having powerand includes at least one rotationally asymmetric reflecting surface;and the transmission optical system includes a transmitting surfacehaving refractive power and includes at least one aspheric surface.

One or more of the above objects of the present invention are alsoachieved by a magnification projection optical system that guides alight beam from an image display panel to a screen in anupstream-downstream direction, projects the light beam from a directioninclined to a normal of the screen, and forms on the screen a magnifiedversion of an image displayed on the image display panel, themagnification projection optical system including: a transmissionoptical system including a plurality of transmitting surfaces; areflection optical system including a plurality of reflecting surfaces;and a diaphragm, wherein one of the reflecting surfaces, on which alight beam passing through the diaphragm is made incident first, hasnegative power.

One or more of the above objects of the present invention are alsoachieved by a magnification projection optical system that guides alight beam from an image display panel to a screen in anupstream-downstream direction, projects the light beam from a directioninclined to a normal of the screen, and forms on the screen a magnifiedversion of an image displayed on the image display panel, wherein aposition and a shape of an intermediate image of the image display panelformed with negative magnification by light beams traveling from theimage display panel to the screen are substantially identical to aposition and a shape, respectively, of an intermediate image of thescreen formed with negative magnification by light beams traveling fromthe screen to the image display panel.

The above-described magnification projection optical systems can projectthe image on the image display panel as a large screen with littledistortion.

One or more of the above objects of the present invention are alsoachieved by a magnification projection apparatus that: displays an imageon an image display panel; illuminates the image display panel withlight from a light source; and by a magnification projection opticalsystem, guides a light beam from the image display panel to a screen inan upstream-downstream direction, projects the light beam from adirection inclined to a normal of the screen, and forms on the screen amagnified version of the image displayed on the image display panel,wherein: the magnification projection optical system comprises areflection optical system and a transmission optical system; thereflection optical system includes a plurality of reflecting surfaceshaving power and includes at least one rotationally asymmetricreflecting surface; and the transmission optical system includes atransmitting surface having refractive power and includes at least oneaspheric surface.

One or more of the above objects of the present invention are alsoachieved by a magnification projection apparatus that: displays an imageon an image display panel; illuminates the image display panel withlight from a light source; and by a magnification projection opticalsystem, guides a light beam from the image display panel to a screen inan upstream-downstream direction, projects the light beam from adirection inclined to a normal of the screen, and forms on the screen amagnified version of the image displayed on the image display panel,wherein: the magnification projection optical system comprises: atransmission optical system including a plurality of transmittingsurfaces; a reflection optical system including a plurality ofreflecting surfaces; and a diaphragm; and one of the reflectingsurfaces, on which a light beam passing through the diaphragm is madeincident first, has negative power.

One or more of the above objects of the present invention are alsoachieved by a magnification projection apparatus that: displays an imageon an image display panel; illuminates the image display panel withlight from a light source; and by a magnification projection opticalsystem, guides a light beam from the image display panel to a screen inan upstream-downstream direction, projects the light beam from adirection inclined to a normal of the screen, and forms on the screen amagnified version of the image displayed on the image display panel,wherein a position and a shape of an intermediate image of the imagedisplay panel formed with negative magnification by light beamstraveling from the image display panel to the screen are substantiallyidentical to a position and a shape, respectively, of an intermediateimage of the screen formed with negative magnification by light beamstraveling from the screen to the image display panel.

The above-described magnification projection apparatuses can be realizedwith a low-profile configuration.

One or more of the above objects of the present invention are alsoachieved by a projection optical system, including: a first opticalsystem including at least one dioptric system and having positive power;and a second optical system including one or more reflecting surfaceshaving power, the second optical system having positive power as awhole, wherein the first and second optical systems are arranged alongan optical path defining an upstream-downstream direction in an orderdescribed from upstream to downstream on a downstream side of an objectsurface; an object image is temporarily formed as an intermediate image,and thereafter, is formed as a normal image; and with respect to anoptical axis of an optical element positioned at a furthest upstream endof the first optical system and having refractive power, at least one ofother optical elements is shifted or tilted.

One or more of the above objects of the present invention are alsoachieved by a projection optical system, including: a first opticalsystem including at least one dioptric system and having positive power;and a second optical system including one or more reflecting surfaceshaving power, the second optical system having positive power as awhole, wherein the first and second optical systems are arranged alongan optical path defining an upstream-downstream direction in an orderdescribed from upstream to downstream on a downstream side of an objectsurface; an object image is temporarily formed as an intermediate image,and thereafter, is formed as a normal image; and in the first opticalsystem, with respect to an optical axis of one of optical elements ofthe first optical system, which one is positioned at a furthest upstreamend of the first optical system and has refractive power, the otheroptical elements are prevented from being tilted.

According to the above-described projection optical systems, the firstoptical system forms an intermediate image, and the second opticalsystem magnifies and projects the intermediate image, thereby increasingthe (enlarging) magnification of the composition of the optical systems.Since the first optical system includes a dioptric system, chromaticaberration is correctable using the chromatic dispersion characteristiceven in the case of employing a color composite prism. Further, bycausing lens elements forming the dioptric system to be shifted ortilted, reverse distortion can be generated effectively in theintermediate image so as to prevent distortion from occurring in theprojected image.

One or more of the above objects of the present invention are alsoachieved by an image projection apparatus that, by a projection opticalsystem, guides a light beam from an image display panel to a screen andforms on the screen a normal version of the image displayed on the imagedisplay panel, wherein: the projection optical system includes: a firstoptical system including at least one dioptric system and havingpositive power; and a second optical system including one or morereflecting surfaces having power, the second optical system havingpositive power as a whole; the first and second optical systems arearranged along an optical path defining an upstream-downstream directionin an order described from upstream to downstream on a downstream sideof an object surface; an object image is temporarily formed as anintermediate image, and thereafter, is formed as a normal image; andwith respect to an optical axis of an optical element positioned at afurthest upstream end of the first optical system and having refractivepower, at least one of other optical elements is shifted or tilted.

One or more of the above objects of the present invention are alsoachieved by an image projection apparatus that, by a projection opticalsystem, guides a light beam from an image display panel to a screen andforms on the screen a normal version of the image displayed on the imagedisplay panel, wherein: the projection optical system includes: a firstoptical system including at least one dioptric system and havingpositive power; and a second optical system including one or morereflecting surfaces having power, the second optical system havingpositive power as a whole; the first and second optical systems arearranged along an optical path defining an upstream-downstream directionin an order described from upstream to downstream on a downstream end ofan object surface; an object image is temporarily formed as anintermediate image, and thereafter, is formed as a normal image; and inthe first optical system, with respect to an optical axis of one ofoptical elements of the first optical system, which one is positioned ata furthest upstream end of the first optical system and has refractivepower, the other optical elements are prevented from being tilted.

The above-described image projection apparatuses can perform extremelyclose range projection with a desired (enlarging) magnification.

One or more of the above objects of the present invention are alsoachieved by a projection optical system guiding and projecting a lightbeam from a projected object surface onto a projection surface in anupstream-downstream direction through a transmission dioptric system anda reflection dioptric system of one or two reflecting mirrors, wherein:the transmission dioptric system includes a plurality of transmissionrefractive elements; substantial telecentricity is provided from theprojected object surface up to a first surface of the transmissiondioptric system; an intermediate image surface of the projected objectsurface is positioned closer to the reflection dioptric system than tothe transmission dioptric system, and an intermediate image on theintermediate image surface is formed as a normal image on the projectionsurface via the reflecting mirrors; the reflecting mirrors include atleast one anamorphic polynomial free-form surface having differentvertical and lateral powers; a light beam from the reflection dioptricsystem to the projection surface is guided at an angle to a normal ofthe projection surface; and the transmission dioptric system isdecentered with respect to a normal of the projected object surface, andthe transmission refractive elements of the transmission dioptric systemare prevented from being decentered with respect to each other.

One or more of the above objects of the present invention are alsoachieved by a projection optical system guiding and projecting a lightbeam from a projected object surface onto a projection surface in anupstream-downstream direction through a transmission dioptric system anda reflection dioptric system of one or two reflecting mirrors, wherein:the transmission dioptric system includes a plurality of transmissionrefractive elements; substantial telecentricity is provided from theprojected object surface up to a first surface of the transmissiondioptric system; an intermediate image surface of the projected objectsurface is positioned closer to the reflection dioptric system than tothe transmission dioptric system, and an intermediate image on theintermediate image surface is formed as a normal image on the projectionsurface via the reflecting mirrors; the reflecting mirrors include atleast one anamorphic polynomial free-form surface having differentvertical and lateral powers; a light beam from the reflection dioptricsystem to the projection surface is guided at an angle to a normal ofthe projection surface; and the transmission dioptric system isdecentered with respect to a normal of the projected object surface, andthe transmission refractive elements of the transmission dioptric systemare prevented from being decentered with respect to each other at agroup unit level.

The above-described projection optical systems can project a largescreen without distortion.

One or more of the above objects of the present invention are alsoachieved by an image projection apparatus magnifying an image displayedon a projected object surface and projecting the magnified image on aprojection surface by a projection optical system, wherein: theprojection optical system guides and projects a light beam from theprojected object surface onto the projection surface in anupstream-downstream direction through a transmission dioptric system anda reflection dioptric system of one or two reflecting mirrors; thetransmission dioptric system includes a plurality of transmissionrefractive elements; substantial telecentricity is provided from theprojected object surface up to a first surface of the transmissiondioptric system; an intermediate image surface of the projected objectsurface is positioned closer to the reflection dioptric system than tothe transmission dioptric system, and an intermediate image on theintermediate image surface is formed as a normal image on the projectionsurface via the reflecting mirrors; the reflecting mirrors include atleast one anamorphic polynomial free-form surface having differentvertical and lateral powers; a light beam from the reflection dioptricsystem to the projection surface is guided at an angle to a normal ofthe projection surface; and the transmission dioptric system isdecentered with respect to a normal of the projected object surface, andthe transmission refractive elements of the transmission dioptric systemare prevented from being decentered with respect to each other.

One or more of the above objects of the present invention are furtherachieved by an image projection apparatus magnifying an image displayedon a projected object surface and projecting the magnified image on aprojection surface by a projection optical system, wherein: theprojection optical system guides and projects a light beam from theprojected object surface onto the projection surface in anupstream-downstream direction through a transmission dioptric system anda reflection dioptric system of one or two reflecting mirrors;

the transmission dioptric system includes a plurality of transmissionrefractive elements;

substantial telecentricity is provided from the projected object surfaceup to a first surface of the transmission dioptric system;

an intermediate image surface of the projected object surface ispositioned closer to the reflection dioptric system than to thetransmission dioptric system, and an intermediate image on theintermediate image surface is formed as a normal image on the projectionsurface via the reflecting mirrors; the reflecting mirrors include atleast one anamorphic polynomial free-form surface having differentvertical and lateral powers; a light beam from the reflection dioptricsystem to the projection surface is guided at an angle to a normal ofthe projection surface; and the transmission dioptric system isdecentered with respect to a normal of the projected object surface, andthe transmission refractive elements of the transmission dioptric systemare prevented from being decentered with respect to each other at agroup unit level.

As will be appreciated, the above-described image projection apparatusescan be formed with a low-profile configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram for illustrating a projection optical system and animage projection apparatus according to an embodiment of the presentinvention;

FIG. 2 is a diagram for illustrating the projection optical system ofFIG. 1 according to the present invention;

FIG. 3 is a diagram for illustrating a configuration of the projectionoptical system of FIG. 1 according to the present invention;

FIG. 4 is a diagram for illustrating a configuration of the projectionoptical system of FIG. 1 according to the present invention;

FIG. 5 is a diagram for illustrating a projection optical systemaccording to an embodiment of the present invention;

FIG. 6 is a diagram for illustrating a magnification projection opticalsystem according to an embodiment of the present invention;

FIG. 7 is a diagram for illustrating an image projection apparatusaccording to an embodiment of the present invention;

FIG. 8 is an enlarged view of a projection optical system of the imageprojection apparatus of FIG. 7 according to the present invention;

FIG. 9 is a diagram for illustrating an image projection apparatusaccording to an embodiment of the present invention;

FIG. 10 is an enlarged view of a projection optical system of the imageprojection apparatus of FIG. 9 according to the present invention;

FIG. 11 is a diagram showing the state of image distortion on a screenaccording to an embodiment of the present invention;

FIG. 12 is a diagram for illustrating an image projection apparatusaccording to an embodiment of the present invention;

FIG. 13 is a diagram for illustrating an image projection apparatusaccording to an embodiment of the present invention;

FIG. 14 is a diagram for illustrating MTF evaluation points on a screenaccording to an embodiment of the present invention;

FIG. 15 is a graph showing MTF characteristics according to anembodiment of the present invention;

FIG. 16 is a graph showing MTF characteristics according to anembodiment of the present invention; and

FIG. 17 is a graph showing MTF characteristics according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description is given below, with reference to the accompanyingdrawings, of embodiments of the present invention.

FIG. 1 is a schematic diagram showing part of an image projectionapparatus according to an embodiment of the present invention.

Referring to FIG. 1, the image projection apparatus includes a lightbulb15, which is a liquid crystal panel in this embodiment. The lightbulb 15is hereinafter referred to simply as a panel 15. The image projectionapparatus further includes a light source 10 composed of a lightemitting part 11 and an illumination optical system 12. The lightemitting part 11 is composed of a lamp and a reflector. A light beamfrom the light emitting part 11 is illumination light in theillumination optical system 12. The illumination light from the lightsource 10 illuminates the panel 15.

An image is formed on the panel 15 in accordance with a modulatingsignal, and the formed image subjects the light beam from the lightsource 10 to two-dimensional intensity modulation, and transmits theintensity-modulated light beam. The light beam transmitted through thepanel 15 is projected and focused onto a screen 21 by a projectionoptical system composed of a first optical system 17 and a secondoptical system 19, so that a magnified version of the image formed onthe panel 15 is displayed.

In light transmission, the panel 15 side may be referred to as theupstream side and the screen 21 side may be referred to as thedownstream side.

FIG. 2 is a diagram for illustrating the projection optical system ofthe image projection apparatus of FIG. 1.

Referring to FIG. 2, the first and second optical systems 17 and 19 areprovided in the order described from upstream to downstream on theprojection or downstream side of the panel 15. The first optical system17 is a dioptric system (lens) having positive power. The second opticalsystem 19 includes a reflecting surface having power, and has positivepower.

The image formed by the panel 15 is formed as an intermediate imageI_(int) in the optical path of the first and second optical systems 17and 19. A magnified version of the intermediate image I_(int) isprojected and formed on the screen 21.

The first optical system 17 is shown as a single lens in FIG. 2.Specifically, however, various modes including a dioptric system, suchas a configuration of a plurality of lenses, a combination of a lens anda mirror, and the configuration of integrated reflecting and refractingsurfaces, are also applicable to the first optical system 17.

The first optical system 17 has positive power as a whole. As shown inFIG. 2, the intermediate image I_(int) formed by the first opticalsystem 17 is an inverted version of the image formed on the panel 15.The intermediate image I_(int) is preferably a one to several timesmagnified image of the image on the panel 15. If the intermediate imageI_(int) is a reduced image, the second optical system 19 is required tohave high enlarging magnification so that the first and second opticalsystems 17 and 19 as a whole obtain a displayed image of highmagnification. This makes it difficult to realize a balance betweenaberration correction and high magnification.

On the other hand, if the enlarging magnification of the intermediateimage I_(int) becomes too high, the second optical system 19 needs to belarger in size, thus increasing the projection optical system andconsequently the image projection apparatus in size.

FIG. 2 shows the optical paths of light beams traveling from positions Aand B on the panel 15 corresponding to a maximum image height point onthe plus (+) side (a position a in FIG. 1) and a maximum image heightpoint on the minus (−) side (a position b in FIG. 1), respectively,toward the positions a and b.

The intermediate image I_(int) is not necessarily required to be formedon a plane. It is sufficient to ensure the performance of the compositeoptical system of the first and second optical systems 17 and 19 as awhole so that a satisfactory image is projected onto the screen 21 bythe composite optical system. Accordingly, no particular restriction isimposed on the imaging performance by the first optical system 17.

According to the configuration of FIGS. 1 and 2, each light beam formingthe intermediate image I_(int) by the first optical system 17 isreflected by the second optical system 19 to have its optical pathfolded back so that an image is projected in the direction opposite tothe direction in which the light beam forming the intermediate imageI_(int) travels.

According to the configuration of FIGS. 1 and 2, the second opticalsystem 19 is composed of a single-surface concave mirror. Alternatively,the second optical system 19 may include two or more reflectingsurfaces. Further, the second optical system 19 may also include adioptric system together with a reflecting surface.

In the configuration of FIGS. 1 and 2, a single reflecting surface maybe added to the second optical system 19 so as to reverse the directionof each finally projected light beam of FIG. 1. Further, in theconfiguration of FIGS. 1 and 2, a dioptric system (lens system) may beprovided as part of the second optical system 19 between the reflectingsurface thereof and the position where the intermediate image I_(int) isformed so as to capture quantity of light more efficiently onto thereflecting surface of the second optical system 19.

As shown in FIG. 2, the light beam starting at the position A on thepanel 15 converges to have a center of gravity at a position A′ in theintermediate image I_(int). The light beam after the convergencediverges at an angle of divergence equal to a collection angle, and isreflected from the second optical system 19 having positive power to befocused onto the position a on the screen 21 in FIG. 1.

Likewise, the light beam starting at the position B on the panel 15converges to have a center of gravity at a position B′ in theintermediate image I_(int). The light beam after the convergencediverges at an angle of divergence equal to a collection angle, and isreflected from the second optical system 19 having positive power to befocused onto the position b on the screen 21 in FIG. 1.

By forming the intermediate image I_(int), it is possible to locallynarrow the effective region of the reflecting surface of the secondoptical system 19, the effective region contributing to the imageformation of the light beams traveling from the positions A and B on thepanel 15. That is, as shown in FIG. 2, the imaging performance for thelight beam traveling from the position A is affected by the figure of areflecting region A″ of the second optical system 19, and the imagingperformance for the light beam traveling from the position B is affectedby the figure of a reflecting region B″ of the second optical system 19.

Accordingly, the configuration of FIGS. 1 and 2 can optimize the figureof each of the reflecting regions A″ and B″. Inversely, it is possibleto control characteristics of light collection to each part on thescreen 21 by locally changing the figure of the concave surface of thesecond optical system 19.

Particularly, its effect can be maximized by providing the concavesurface with a free-form surface figure.

Optimum data setting may be performed by a conventionally knownsimulation method such as ray tracing. Since the optimization can beperformed based on the reflecting surface, it is possible to performsuch design as to prevent the generation of or an increase in chromaticaberration and at the same time improve other light collectioncharacteristics.

Since the first optical system 17 includes a dioptric system, it ispossible to correct chromatic aberration, which is uncorrectable by onlya reflecting surface, by the dioptric system.

Further, aberrations that affect the image projected on the screen 21and cannot be corrected by only the reflecting surface of the secondoptical system 19 may be corrected by actively incorporating a dioptricsystem in the second optical system 19.

In the configuration of FIG. 1, the imaging magnification of the secondoptical system 19 may be increased by bringing the position where theintermediate image I_(int) is formed closer to the reflecting surface ofthe second optical system 19. This is described below with reference toFIGS. 3 and 4.

In FIGS. 3 and 4, reference numeral 15 denotes the panel, referencenumeral 17A denotes a first optical system (a lens), reference numeral19A denotes a second optical system (a concave mirror), and referencenumeral 21 denotes the screen.

In accordance with the notation of optical system data, the radius ofcurvature of the i^(th) surface from the panel 15 is referred to as Ri(i=1 through 3; the first surface (i=1) is the entrance surface of thefirst optical system 17A; the third surface (i=3) is the reflectingsurface of the second optical system 19A), and the surface separationbetween the i^(th) surface and the (i+1)^(th) surface is referred to asTi (i=0 through 3; the 0^(th) surface separation (i=0) is the distancebetween the panel 15 and the entrance surface of the first opticalsystem 17A; the third surface separation (i=3) is the distance betweenthe second optical system 19A and the screen 21).

The data of the optical system of FIG. 3 is as follows:

i Ri (mm) Ti (mm) Material 0 85 1 65 25 BK7 2 −55 225 3 −135 −400

An object height on the panel 15 is ±7.5 mm.

When a setting is provided such that the projected image is optimallyformed at an image height position 0 (a point P on the screen 21), thedistance between principal rays starting from the positions A and B onthe panel 15 that is an object and reaching the positions a and b on thescreen 21, respectively, is approximately 208 mm.

On the other hand, in FIG. 4, a first optical system 17B and a secondoptical system 19B are employed. The power of the first optical system17B is reduced so that the position of the intermediate image I_(int)moves away from the first optical system 17B. At the same time, thepositive power of the second optical system 19B is controlled whilemaintaining the positional relationship so that light collectioncharacteristics at the image height are maintained. The data of theoptical system in this case is as follows.

i Ri′ (mm) Ti′ (mm) Material 0 85 1 65 25 BK7 2 −60 225 3 −98 −400

An object height on the panel 15 is ±7.5 mm.

The optical arrangement of FIG. 4 is drawn differently from that of FIG.3 for convenience of description. As is apparent from theabove-described data, however, the optical arrangement of FIG. 4 issubstantially equal to that of FIG. 3. In the optical arrangement ofFIG. 4, the curvature of the exit surface of the first optical system17B and the curvature of the reflecting surface of the second opticalsystem 19B are different from those of the first and second opticalsystems 17A and 19A, respectively.

In the case of FIG. 4, the distance between principal rays starting fromthe positions A and B on the panel 15 and reaching positions a′ and b′on the screen 21, respectively, is approximately 362 mm. In the case ofFIG. 4, the magnification increases compared with the case of FIG. 3,where the distance between principal rays is 208 mm. That is, thepositive power of the first optical system 17B is reduced and theintermediate image I_(int) is closer to the reflecting surface havingpositive power of the second optical system 19B, thus resulting in anincreased magnification. As described above, a magnification can beincreased by only changing the radius of curvature of each of arefracting surface and a reflecting surface without changing an opticalarrangement.

The above-described matter may be implemented by providing, on thelightbulb side of the intermediate image (the side upstream of theintermediate image in the upstream-downstream direction from the panel15 to the screen 21) in the optical path of the first and second opticalsystems, an optical element having negative power for bringing theposition at which the intermediate image is formed close to thereflecting surface having positive power of the second optical system.

As the above-described negative-power optical element, a concave lens, aFresnel concave lens, a convex reflecting mirror, or a composite systemthereof may be employed, for instance.

Actually, it is necessary to ensure light collection (condensing)characteristics at each image height position and to correct thedistortion of an image surface. The latitude of design may be increasedby increasing the number of refracting and reflecting surfaces so thatdesign optimization may be performed by a conventionally knownsimulation such as ray tracing.

FIG. 5 is a diagram showing an embodiment according to the presentinvention. In FIG. 5, the same elements as those of FIG. 1 are referredto by the same numerals.

In the embodiment of FIG. 5, a second optical system 190 includes areflecting surface 192 having positive power and a reflecting surface191 having negative power.

Each light beam from the panel 15 becomes an imaging light beam by theaction of the first optical system 17. Before forming the intermediateimage I_(int), each light beam is made incident on the negative-powerreflecting surface 191 to be reflected toward the reflecting surface192. The intermediate image I_(int) is formed between the reflectingsurfaces 191 and 192. The intermediate image I_(int) is furthermagnified by the positive power of the reflecting surface 192 so that animage on the panel 15 is projected onto the screen 21.

That is, the reflecting surface 191 of the second optical system 190 isan example of the above-described negative-power optical element havingthe effect of bringing the position where the intermediate image I_(int)is formed close to the positive-power reflecting surface 192 of thesecond optical element 190.

As the reflecting surface 191, a reflecting optical element havingdivergent power, such as a convex reflecting surface, a Fresnel convexreflecting mirror, or a hologram reflecting mirror having positivepower, may be suitably employed.

The projection optical system slightly increases in size to move theposition of the intermediate image I_(int) away from the lightbulb 15.However, by forming the above-described negative-power optical elementof a reflecting mirror, it is possible to employ a layout folding backan optical path, thereby reducing the size of the entire optical system.

By employing the above-described negative-power optical element forbringing the position where the intermediate image I_(int) is formedclose to the positive-power reflecting surface of the second opticalsystem, it is possible to perform such control as to narrow the degreeof divergence of a light beam incident on the positive-power reflectingsurface of the second optical system. As a result, the effectivereflecting region of the positive-power reflecting surface can bereduced.

Light collection characteristics and distortion can be controlled morefinely by controlling the effective reflecting region of thepositive-power reflecting surface and providing a local figure to thefigure of the reflecting surface, that is, setting a free-form surfacefigure.

The employment of the above-described configuration makes it possible towiden an angle compared with the conventional projection optical system.

Forming at least one of the above-described reflecting surfaces of afree-form surface provides more design latitude, thus making it easy tocorrect aberrations.

The free-form surface is a surface including a non-rotationallysymmetric surface figure, such as an anamorphic surface or an X-Ypolynomial surface.

In design, a desirable image formation characteristic can be realized byforming each of the (refracting and reflecting) surfaces included in theprojection optical system of a free-form surface. Actually, however,this requires stricter accuracy in the relative positional error and thedecentering error of each surface. Accordingly, it is not necessarilythat the larger the number of free-form surfaces, the better. Rather, itis preferable to set an optimum number of free-form surfaces.

As described above, each light beam that has formed the intermediateimage I_(int) becomes a divergent light beam thereafter to be madeincident on the positive-power reflecting surface (concave mirror) ofthe second optical system. Accordingly, a divergent light beam from eachposition of the intermediate image I_(int) is reflected from a localreflecting region on the concave mirror to be focused onto the screen.In other words, a light beam focused onto each position on the screencorresponds to a local reflecting region for each image height on theconcave mirror.

Thus, the surface figure of the concave mirror (a reflecting surfacefrom which a light beam forming the intermediate image is reflectedfirst after the formation of the intermediate image) may be a free-formsurface so that the curved surface figure of the reflecting surface maybe controlled with respect to a reflecting region for each image height.As a result, aberrations can be corrected most effectively, and theperformance can be improved.

The smaller the number of free-form surfaces, the better, consideringsurface processing and assembly. It is most effective to apply afree-form surface preferentially to the positive-power reflectingsurface (concave mirror) immediately after the position where theintermediate image is formed.

It is possible to design so as to increase light collectioncharacteristics and an image distortion characteristic by setting thefigure of a free-form surface that can control a local figure of aconcave surface having light collection (condensing) power as well ascontrolling a reflecting region.

The intermediate image may be a one to several times magnified image. Ahigh imaging magnification is unnecessary for the first optical systemrelating to the formation of the intermediate image. Accordingly, it ispossible to optimize the first optical system by the conventionalconfiguration of only a dioptric system. Forming the first opticalsystem by only a dioptric system makes it easy to optically design thefirst optical system, and makes it possible to relax tolerance forsurface processing and assembly.

Further, it is also possible to increase design latitude by increasingthe number of refracting surfaces and distribute tolerance, therebyimproving performance.

The first optical system may be formed of only a dioptric system.However, if a further improvement in performance is desired, thelatitude in the configuration of the first optical system may be furtherincreased.

In the case of pursuing such a further improvement in performance, thefirst optical system may be formed of a reflecting surface having arotational symmetry axis and a dioptric system. The reflecting surfacehaving a rotational symmetry axis can be formed relatively easily, andis extremely effective in increasing design latitude without losingprocessability and assemblability. A further increase in latitude may beachieved by providing an aspheric surface figure to the reflectingsurface having a rotational symmetry axis. Further, providing latitudeof shift or decentration to the reflecting surface makes it possible todesign with increased latitude.

The dioptric system may also employ an aspheric surface figure. Theemployment of this configuration increases latitude in design, thusmaking it possible to realize a projection optical system with higherperformance.

The reflecting surface may be processed by any of conventionally knownvarious processing methods such as grinding, molding using a metal mold,and accurate shape transfer processing. Further, a refracting andtransmitting surface and a reflecting surface may be integrated so as tohave a total internal reflection structure.

Referring back to FIG. 1, a further description is given of the imageprojection apparatus.

As the lamp of the light emitting part 11 of the light source 10, ahalogen lamp, a xenon lamp, a metal halide lamp, or a super-highpressure mercury lamp may be employed.

The reflector integrated with the lamp to be provided in the vicinitythereof is employed to obtain high illumination efficiency. Although notshown in FIG. 1, a well known illuminance uniforming part referred to asan integrator optical system may be employed to project a light beamreflected from the reflector to have directivity onto the panel 15 withuniform light intensity so that the surface of the panel 15 isilluminated with an even illumination distribution.

In the case of replacing the transmission liquid crystal panel employedas the lightbulb 15 with a reflection liquid crystal lightbulb,efficient illumination can be performed by splitting an illuminationoptical path and a projection optical path using a polarization beamsplitter.

Further, in the case of employing a Digital Micromirror Device (DMD) asa lightbulb, an optical path splitting optical system using an obliqueincidence optical system or a total reflection prism may be employed.Thus, a suitable optical system may be employed in accordance with alightbulb type.

In a front-type projector, it is desirable that a projected image beshifted upward so as not to be hidden behind the projector when viewedfrom a viewer. That is, the lightbulb 15 is shifted (downward in FIG. 1)in a plane perpendicular to the optical axis of the projection opticalsystem (the optical axis of the first optical system 17) so that lightbeams are made incident on the projection optical system from its lowerside.

As the shift of the lightbulb 15 increases, particularly, an effectiveangle of view should be widened as a specification required for thefirst optical system 17. The shift of the lightbulb 15 is set to asuitable size. The intermediate image I_(int) is formed temporarily bythe first optical system 17. The image formed by the lightbulb 15 ismagnified and projected onto the screen 21 by the positive-power secondoptical system 19.

In the case of a projector of a rear projection type, space occupancycan be lowered by folding an optical path by providing a plane mirror ina projection optical path.

In the above-described embodiment, only the single panel 15 is shown forsimplification. Alternatively, three panels for red, green, and blue maybe employed. Light beams modulated by the three panels may be subjectedto color composite by a well known color composite part such as adichroic prism, and thereafter, be made incident on the first opticalsystem 17. Thereby, a color image can be projected onto the screen 21.

The projection optical system according to this embodiment includes thefirst optical system 17 and the second optical system (for instance, thesecond optical system 19). The image formed by the lightbulb 15 isformed as an intermediate image in the optical path of the first andsecond optical systems 17 and 19, and the intermediate image is furthermagnified and projected. Accordingly, high projection magnification canbe realized. Since the first optical system 17 includes a dioptricsystem, chromatic aberration is correctable using the chromaticdispersion characteristic even in the case of employing a colorcomposite prism. Further, since the optical path of imaging light beamsis folded back by a reflecting surface of the second optical system 19,the projection optical system can be made compact.

Accordingly, the image projection apparatus according to thisembodiment, including this projection optical system, can be madecompact with a long optical path of image light beams being provided ina space in the apparatus. As a result, a large-size image can beprojected and displayed with a reduced projection space outside theapparatus.

Next, a description is given of another embodiment according to thepresent invention.

Referring to FIG. 6, the reference ray of a group of light beamstraveling from an image display panel 1 (hereinafter referred to simplyas a panel 1) toward a screen 2 is made incident thereon at apredetermined angle to the normal of the screen 2. The reference ray isthe principal ray of a light beam guided from the center of the panel 1to the screen 2.

The panel 1 is a reflection liquid crystal panel, and is illuminatedwith linearly polarized illumination light via a polarization beamsplitter 10A. Light beams modulated by the panel 1 become imaging lightbeams through the polarization beam splitter 10A. The image displaypanel may be a lightbulb such as a transmission or reflection liquidcrystal panel or a DMD.

In light transmission, the panel 1 side may be referred to as theupstream side and the screen 2 side may be referred to as the downstreamside. A transmission optical system 3 that includes a transmittingsurface having refractive power and at least one aspheric surface isprovided on the downstream side of the panel 1. A reflection opticalsystem including a plurality of reflecting surfaces 4 through 8 isprovided on the downstream side of the transmission optical system 3.

The imaging light beams from the panel 1 are transmitted inside thetransmission optical system 3 to be guided to the screen 2 via thereflecting surfaces 4 through 8 forming the reflection optical system.Of the reflecting surfaces 4 through 8 forming the reflection opticalsystem, the reflecting surface 8 is a rotationally asymmetric reflectingsurface.

It is desirable to provide the transmission optical system 3 with lightbeam condensing action. In this embodiment, a magnification effect loadis reduced in the transmission optical system 3 so as to preventparticularly the aperture of a lens on the downstream side from becominglarger in size. Accordingly, the entire or a substantial portion of themagnification effect as a magnification projection optical system isassumed by the reflection optical system.

Asymmetric aberration (aberration resulting from the vertical asymmetryof the reference axis in FIG. 6) is corrected by the rotationallyasymmetric reflecting surface 8, and the optical axis of thetransmission optical system 3 is set to be decentered with respect tothe panel 1 (in this embodiment, the optical axis is eccentric in theupward direction of FIG. 6 with respect to the center of the panel 1).As a result, the effect of the correction of asymmetric aberration isincreased. That is, the correction of asymmetric aberration is assumedby both the transmission optical system 3 and the reflection opticalsystem.

The entire transmission optical system 3 is assembled coaxially so as tobe formed into a cell easily.

In the case of forming a magnification projection optical system of atransmission optical system and a reflection optical system as in thisembodiment, it is easy to assemble the optical system compared with thecase of forming all optical surfaces of reflecting surfaces. Further, itis also possible to utilize the “optical path folding-back effect” sothat the entire system can be reduced in size.

A diaphragm 9 is provided on the upstream side of the reflecting surface4 on the downstream side of the transmission optical system 3. An imageI9 of the diaphragm 9 is formed with negative reducing magnification inan imaging optical path by the reflecting surfaces 4 through 7 on thedownstream side of the diaphragm 9. That is, the image I9 of reducingmagnification of the diaphragm 9 is formed as an inverted image betweenthe reflecting surface 7 and the rotationally asymmetric reflectingsurface 8 by the action of the reflecting surfaces 4 through 7 of thereflection optical system.

By thus providing a power arrangement such that the image I9 of thediaphragm 9 is formed with reducing magnification, a light beam madeincident on a reflecting surface on the downstream side of the image I9of the diaphragm 9 (the reflecting surface 8 in this embodiment) isprevented from widening greatly. That is, the effective aperture of thereflecting surface can be reduced. As a result, this reflecting surfacecan be reduced in size.

As described above, the image I9 of the diaphragm 9 is formed within theoptical path of the reflection optical system (between the reflectingsurface 7 and the rotationally asymmetric reflecting surface 8). Theposition where the image I9 is formed is a screen (downstream)-sidepupil, that is, the exit pupil of the magnification projection opticalsystem.

The imaging light beams form the intermediate image of the panel 1 inthe optical path inside the reflection optical system. Like the image I9of the diaphragm 9, the intermediate image is a real image of negativemagnification and an inverted image. In the embodiment of FIG. 6, theintermediate image of the panel 1 is formed in the vicinity of thereflecting surface 7. That is, the intermediate image of the panel 1 isformed by the transmission optical system 3 and the reflecting surfaces4 through 6.

It is a magnified image of the intermediate image of the panel 1 by thereflecting surface 7 and the rotationally asymmetric reflecting surface8 that is formed on the screen 2. The imaging magnification at this timeis also negative. Thus, the light beams from the panel 1 form theintermediate image as an inverted image, and the inverted image isinversed to be imaged on the screen 2 as an erect image. At this point,trapezoidal distortion generated in the intermediate image offsetstrapezoidal distortion at the time of forming the image on the screen 2.As a result, the image can be displayed with reduced trapezoidaldistortion.

In the case of forming the transmission optical system 3 into a cell asa single unit, the relative positional adjustment between thetransmission optical system 3 formed into the cell and the reflectionoptical system remains as positional adjustment in the magnificationprojection optical system. At this point, the reflecting surface 4provided furthest on the upstream side in the reflection optical systemmay have positive power. In this case, if the light beams emitted fromthe transmission optical system 3 are condensing light beams, the lightbeams are further condensed by the positive power of the reflectingsurface 4.

In this embodiment, the power of the reflecting surface 4 is negative.If the reflecting surface 4 has positive power, a greater aberration isgenerated when the relative positions of the transmission optical system3 and the reflection optical system are offset. In other words, if thereflecting surface 4 has positive power, a variation in aberration isgreater with respect to the same offset between the transmission opticalsystem 3 and the reflection optical system.

In the case of providing the reflecting surface with negative power asin this embodiment, a variation in aberration with respect to the offsetis small. Accordingly, the accuracy of the relative positionalrelationship between the transmission optical system 3 and thereflection optical system is relaxed, thus facilitating the assemblingof the optical system.

To simplify integrating the transmission optical system 3 and thereflection optical system, the transmission optical system 3 and thereflection optical system may be assembled separately. At this point,the assembling of each of the transmission optical system 3 and thereflection optical system may be accompanied by assembly error.Accordingly, it is difficult to totally eliminate the relativepositional offset between the transmission optical system 3 and thereflection optical system. Given the occurrence of a relative positionaloffset, it is preferable, in terms of tolerance sensitivity, that thereflecting surface 4 having power be a negative-power reflectingsurface.

The light beam emitted from the transmission optical system 3 is rarelydivergent, and normally, is convergent. At this point, if the reflectingsurface 4 has positive power, the light beam is further converged. Onthe other hand, if the reflecting surface 4 has negative power, theconvergence of the light beam is relaxed. Comparing these two cases, avariation in the state of the light beam due to the relative positionaloffset between the transmission optical system 3 and the reflectionoptical system is greater in the former case, thus imposing stricterassembly tolerance. By providing negative power to the reflectingsurface 4, assembly tolerance can be eased.

The reflecting surface 4 is provided with negative power, and thereflecting surface 5, positioned on the downstream side of thereflecting surface 4, is provided with positive power. A succession ofnegative-power reflecting surfaces on the upstream side in thereflection optical system causes incident light beams to be overlydivergent, thus preventing the diaphragm image from being formed in thereflection optical system.

It is preferred to provide the reflecting surface 5 with positive powerin order to provide a tendency to converge to the imaging light beamsreflected from the reflecting surface 5 so that the image I9 of thediaphragm 9 is formed in the optical path of the reflection opticalsystem. That is, the composite power of the optical system (reflectingsurfaces 4 through 7) provided between the diaphragm 9 and the image I9thereof is positive. Providing positive power to the reflecting surface5 makes it possible to prevent the separation of light beams ofdifferent angles of view and reduce the size of the reflecting surfacereceiving the light beams.

It is also possible to provide the reflecting surface 5 as well as thereflecting surface 4 with negative power and the reflecting surface 6 onthe downstream side of the reflecting surface 5 with positive power. Inthis case, however, it may be necessary to increase the power of thereflecting surface 6. Further, an inter-surface distance may be enlargedto increase aberration to be generated, enlarge the optical system, andthus increase the number of surfaces forming the optical system, whichcase begins to lose merit.

According to the present invention, in the magnification projectionoptical system where each of the panel 1 and the screen 2 is a planesurface and “oblique projection” is performed, optical systems areformed on the upstream side and the downstream side, respectively, of anintermediate image plane so that the position and the shape of thenegative-magnification intermediate image of the panel 1 generated bylight beams traveling from the panel 1 side toward the screen 2 (theintermediate image of the panel 1 generated in the optical path of animaging light beam by the magnification projection optical system) aresubstantially identical to those of the negative-magnificationintermediate image of the screen 2 generated by light beams travelingfrom the screen 2 side toward the panel 1. Thereby, the sum of thedistortion generated between the panel 1 and the intermediate image andthe distortion generated between the intermediate image and the screen 2can be approximated to zero. Accordingly, it is possible to form animage on the screen 2 with reduced distortion.

The light beams traveling from the screen 2 side toward the panel 1 areimaginary light beams used in performing ray tracing with the screen 2being an object surface and the panel 1 being an image surface indesigning the magnification projection optical system. In this case, theintermediate image of the screen 2 in the optical path is formed as areduced image on the panel 1.

It is preferable that the rotationally asymmetric reflecting surface 8be positioned closest to the screen 2, provided furthest on thedownstream side, in the optical path of the imaging light beams in thereflection optical system as in this embodiment. The reflecting surfaces4 through 8 are provided so that the overlap area of the incidentpositions of the light beams corresponding to different angles of viewincreases toward the upstream direction and decreases toward thedownstream direction.

On a rotationally asymmetric reflecting surface, a figure is formablewith high latitude with respect to an incident position. Accordingly, bypositioning the rotationally asymmetric reflecting surface 8 furthest onthe downstream side and reducing the overlap area of the light beamscorresponding to different angles of view at this surface position, therotationally asymmetric reflecting surface 8 can be provided with afigure suitable for the correction of the residual aberration of thelight beams of the angles of view generated by the optical system on theupstream side of the rotationally asymmetric reflecting surface 8.Thereby, highly effective aberration correction can be realized. Thatis, the residual aberration of the light beams of respective imageheight positions reaching the last reflecting surface 8 can be correctedeasily by providing the reflecting surface 8 with suitable figures forthe respective incidence positions of the light beams.

If a rotationally asymmetric reflecting surface is provided on theupstream side of the reflection optical system, light beams of differentangles of view are incident on the same position on the reflectingsurface, overlapping with each other. Accordingly, it is difficult toobtain a figure solution that simultaneously corrects the aberrations ofthe light beams of the different angles of view.

According to this embodiment, the transmission optical system 3 may alsotake partial charge of the correction of an asymmetric aberrationcomponent. In this case, the transmission optical system 3 is providedwith a rotationally asymmetric transmitting surface in order to increasethe effect of the correction. A rotationally asymmetric surface iseffective in correcting an aberration component that is difficult for arotationally symmetric aspheric surface to correct. That is, therotationally asymmetric transmitting surface can generate aberrationthat cannot be generated by a rotationally symmetric transmittingsurface. The aberration thus generated can be used to cancel otheraberration.

The reflecting surfaces 4 through 8, forming the reflection opticalsystem, may be formed integrally with one another as a unit. This makesit easy to provide accuracy to the relative positions of the reflectingsurfaces 4 through 8 and assemble the magnification projection opticalsystem. The reflecting surfaces 4 through 8 may be integrated by, forinstance, molding. The reflecting surfaces 4 through 8 may also beintegrated by another suitable method, as will be appreciated.

The position of the diaphragm 9 is not limited to the position of FIG.6. The diaphragm 9 may be provided between surfaces in the transmissionoptical system 3, for instance. In this case, part of the transmissionoptical system 3 contributes to the formation of the image I9 of thediaphragm 9.

The number of image display panels is not limited to one. Three imagedisplay panels may be employed so that images of R (red), G (green), B(blue) color components may be displayed on the different image displaypanels corresponding to their respective colors. Light beams from theimage display panels may be composed to be guided to a screen by themagnification projection optical system so that a color image isdisplayed on the screen.

In this case, a combination of a polarization beam splitter and adichroic prism (well known in color image projection apparatuses) may beused between the panel 1 and the transmission optical system 3 in FIG.6.

Further, the screen is not necessarily a plane surface.

As described above, according to the magnification projection opticalsystem of FIG. 6, the light beams from the panel 1 are guided to thescreen 2 and projected thereonto from a direction inclined to the normalof the screen 2 so that a magnified version of an image displayed on thepanel 1 is formed on the screen 2. The magnification projection opticalsystem includes the reflection optical system and the transmissionoptical system 3. The reflection optical system is formed of thereflecting surfaces 4 through 8 having power. The reflecting surface 8is a rotationally asymmetric reflecting surface. The transmissionoptical system 3 includes a transmitting surface having refractive powerand at least one aspheric surface. The transmission optical system 3 mayinclude a Fresnel lens surface. The reflection optical system may beconfigured so that light reflection on the reflecting surfaces of thereflection optical system satisfies an internal total reflectioncondition. In the case of the reflecting surfaces being internal totalreflection surfaces, the surface that captures a light beam from thetransmission optical system is a transmission surface. In this case, itis preferable that the incident light beam be perpendicular to thetransmitting surface because this prevents aberration from occurring atthe time of the incidence.

According to the above-described magnification projection opticalsystem, the (imaging) light beams from the panel 1 are projected ontothe screen 2 from a direction inclined to the normal of the screen 2 soas to form a magnified image on the screen 2. This effectively reducesdistortion that occurs when imaging light beams projected from thedirection of the normal of the screen 2 are inclined thereto.

The transmitting surface having refractive power of the transmissionoptical system 3 may be realized easily by a lens system, and may alsobe formed into a cell easily. Therefore, it is easy to provide assemblyaccuracy, thus producing the effect of cost reduction.

According to the above-described magnification projection opticalsystem, an asymmetric aberration component is correctable by therotationally asymmetric reflecting surface 8.

If the magnification projection optical system is composed of onlyrefracting surfaces, the array of the reflecting surfaces extends in onedirection. Therefore, it is impossible to reduce the size of thethree-dimensional structure of the optical system. However, by combininga transmitting surface and a reflecting surface, the optical path can befolded so that the optical system can be reduced in size.

For instance, by providing the optical path of a transmission opticalsystem parallel to a screen, and folding the optical path to the screenside on the image side of the transmission optical system, the opticalsystem can be reduced in thickness while maintaining the same opticalpath length.

The diaphragm 9 is provided between the first surface of thetransmission optical system 3 from its image display panel side and thefirst surface of the reflection optical system from its screen side. Theimage I9 of the diaphragm 9 is formed with negative reducingmagnification by the reflecting surfaces (optical elements) 4 through 7provided on the screen side.

Of the reflecting surfaces 4 through 8 of the reflection optical system,the reflecting surface 4, which has power and on which a light beampassing through the diaphragm 9 is made incident first, has negativepower. The reflecting surface 5, subsequent to the reflecting surface 4,has positive power. Of the reflecting surfaces 4 through 8 forming thereflection optical system, the reflecting surface 8 is a rotationallyasymmetric reflecting surface. The transmission optical system 3includes a transmitting surface having refractive power and at least oneaspheric surface.

According to the magnification projection apparatus of FIG. 6, theposition and the shape of the intermediate image with negativemagnification of the panel 1 generated by light beams traveling from thepanel 1 to the screen 2 are substantially identical to those of theintermediate image with negative magnification of the screen 2 generatedby light beams traveling from the screen 2 to the panel 1.

The rotationally asymmetric reflecting surface 8 is positioned at thefurthest downstream end of the second optical system in the projectionoptical path. The transmission optical system 3 includes a rotationallyasymmetric transmitting surface having refractive power. The opticalaxis of the transmission optical system 3 is set in a plane including alight guide path so as to be decentered with respect to the position ofthe panel 1. Thereby, aberration reverse to the aberration generated ona decentered reflecting surface in the reflection optical system can begenerated so that it is possible to cause the aberrations to cancel eachother. The light guide path is the optical path of a principal ray fromthe center of the panel 1 to the center of the magnified image on thescreen 2 in the optical path from the panel 1 to the screen 2. Further,the reflection optical system can be formed as a unit.

Accordingly, by adding any of various well known light sources to themagnification projection optical system of FIG. 6, a magnificationprojection apparatus that displays an image on the panel 1, illuminatesthe panel 1 with light from the light source, guides a light beam fromthe illuminated panel 1 to the screen 2 by the magnification projectionoptical system, projects the light beam from a direction inclined to thenormal of the screen 2, and projects a magnified version of the imagedisplayed on the panel 1 onto the screen 2 can be realized.

Thus, the magnification projection optical system according to thisembodiment can project the image on the image display panel 1 as a largescreen with little distortion. Further, the magnification projectionapparatus including this magnification projection optical system can berealized with a low-profile configuration.

Next, a description is given of another embodiment according to thepresent invention.

FIG. 7 is a diagram showing an image projection apparatus including aprojection optical system according to this embodiment. FIG. 8 is anenlarged view of the projection optical system. The definition of“upstream” and “downstream” in an optical path of the above-describedembodiments also applies to this and the following embodiments.

Referring to FIG. 8, the projection optical system includes a firstoptical system 71 provided on the object or upstream side and a secondoptical system 72 provided on the image or downstream side. The firstoptical system 71 is formed of lenses 711 through 716, and includes adiaphragm S immediately after the lens 713. The lens 713 is a doublet.The second optical system 72 includes reflecting surfaces 721 and 722.

On the object side of the first optical system 71, it is assumed, asdescribed with reference to FIG. 6, that a reflection liquid crystalpanel is illuminated with linearly polarized illumination light via apolarization beam splitter so that reflected light beams modulated bythe liquid crystal panel become imaging light beams through thepolarization beam splitter. Referring to FIG. 8, PB denotes thepolarization beam splitter.

Light beams from the object side project an image on a projectionsurface (a screen in FIG. 7) via the first and second optical systems 71and 72. The intermediate image of the object is formed between thereflecting surfaces 721 and 722, and is formed on the projection surfaceas a normal image by the reflecting surface 722.

As the object displaying an image to be projected, one configured toilluminate the lightbulb 15 with a light beam from the light emittingpart 11 formed of the lamp and the reflector through the illuminationoptical system 12, as described with reference to FIG. 1, may beemployed. Specifically, a halogen lamp, a xenon lamp, a metal halidelamp, or a super-high pressure mercury lamp is suitable as the lightemitting part 11. An integrator optical system that makes the intensityof the light beam reflected from the reflector to have directivityuniform with respect to the lightbulb 15 may be employed as theillumination optical system 12.

As the above-described object, a type of object that performs opticalpath splitting with respect to a DMD panel using an oblique incidenceoptical system or a total reflection prism may be employed. A type ofimage display device illuminated with light from an external lightsource, such as a lightbulb such as a liquid crystal panel, a DMD, or afilm slide, may also be employed as the above-described object. Anobject of a self-luminous type, such as a two-dimensional arrangement oflight-emitting diodes, an LED array, an EL array, or a plasma display,may also be employed.

The projection optical system shown in FIGS. 7 and 8 includes thepositive-power first optical system 71 including at least one dioptricsystem (such as the lens 711) and the second optical system 72 havingpositive power as a whole and including at least one reflecting surfacehaving power (such as the reflecting surface 721). The first and secondoptical systems 71 and 72 are provided in the order described from theobject surface side so that an object image is formed temporarily as anintermediate image, and thereafter, is formed as a normal image. In thefirst optical system 71, with respect to the optical axis of the lens(optical element) 711, positioned furthest on the object side (closestto the object) and having refractive power, the other optical elements,or the lenses 712 through 716 and the reflecting surfaces 721 and 722,are shifted or tilted. That is, a shift or tilt is caused in units ofoptical elements. The dioptric system may include a light transmittingtype element performing diffraction.

According to the projection optical system according to this embodiment,the first optical system 71 forms an intermediate image, and the secondoptical system 72 magnifies and projects the intermediate image, therebyincreasing the (enlarging) magnification of the composition of theoptical systems 71 and 72. Since the first optical system 71 includes adioptric system, chromatic aberration is correctable using the chromaticdispersion characteristic even in the case of employing a colorcomposite prism. Further, by causing lens elements forming the dioptricsystem to be shifted or tilted, reverse distortion can be generatedeffectively in the intermediate image so as to prevent distortion fromoccurring in the projected image. Accordingly, the image projectionapparatus according to this embodiment can perform extremely close rangeprojection with a desired (enlarging) magnification.

Next, a description is given of another embodiment according to thepresent invention.

FIG. 9 is a diagram showing an image projection apparatus including aprojection optical system according to this embodiment. FIG. 10 is anenlarged view of the projection optical system.

Referring to FIG. 10, reference numeral 100 denotes an object-side partof a type that illuminates a reflection liquid crystal panel withlinearly polarized illumination light via a polarization beam splitterso that reflected light beams modulated by the liquid crystal panelbecome imaging light beams through the polarization beam splitter asdescribed above with reference to FIG. 6. The image display surface ofthe reflection liquid crystal panel is a projected object surface (asurface on which an image to be projected is displayed as an object). InFIG. 10, PB denotes the polarization beam splitter. The object-sideconfiguration is not limited to this. The configuration described abovewith reference to FIG. 1 where the lightbulb 15 is illuminated with alight beam from the light emitting part 11 formed of the lamp and thereflector through the illumination optical system 12 may also beemployed. This configuration may include an integrator optical system.Further, a configuration that performs optical path splitting withrespect to a DMD panel using an oblique incidence optical system or atotal reflection prism may be employed. The projected object surface maybe realized by a type of image display device illuminated with lightfrom an external light source, such as a lightbulb such as a liquidcrystal panel, a DMD, or a film slide. Alternatively, an object of aself-luminous type, such as a two-dimensional array of light emittingdiodes, an LED array, an EL array, or a plasma display, may also beemployed.

The projection optical system of FIG. 10 includes a transmissiondioptric system 120 and a reflection dioptric system 130.

The transmission dioptric system 120 includes lenses 121 through 127.The lenses 123 and 126 are doublets. In the illustrated exemplaryembodiment, the transmission dioptric system 120 is formed of ninelenses. The reflection dioptric system 130 includes a first reflectingmirror 131 and a second reflecting mirror 132. The first and secondreflecting mirrors 131 and 132 are arranged in the order described fromupstream to downstream on the downstream side of the transmissiondioptric system 120. FIG. 10 shows only the reflecting surfaces of thereflecting mirrors 131 and 132. The reflecting mirrors 131 and 132 aregeneral optical elements performing reflection and refraction of lightat the interface of reflection. Alternatively, the reflecting mirrors131 and 132 may be light reflecting optical elements performingdiffraction.

That is, according to the projection optical system shown in FIGS. 9 and10, light beams from the projected object surface are guided by thetransmission dioptric system 120 and the reflection dioptric system 130to be projected onto a projection surface (a screen shown in FIG. 9).

The transmission dioptric system 120 includes the lenses (transmissionrefractive elements) 121 through 127. It is substantially telecentricfrom the projected object surface up to the first surface (theobject-side surface of the lens 121) of the transmission dioptric system120 as shown in FIG. 10. The intermediate image surface of the projectedobject surface is positioned between the reflecting mirrors 131 and 132of the reflection dioptric system 130. An intermediate image on theintermediate image surface is re-formed as a normal image on theprojection surface via the second reflecting mirror 132. Thetransmission refractive element means a general optical elementperforming refraction of light at the interface of a light transmittingmedium, and is typically a lens. Alternatively, the transmissionrefractive element may be a light transmitting element performingdiffraction.

As shown in Embodiment 6 described below, the first reflecting mirror131 has negative-power and is axially symmetric, and the secondreflecting mirror 132 is an anamorphic polynomial free-form surfacewhose vertical power and lateral power are different. A light beam fromthe second reflecting mirror 132 is guided to the projection surface atan angle to the normal of the projection surface.

The transmission dioptric system 120 is decentered with respect to thenormal of the projected object surface. The transmission refractiveelements 121 through 127 of the transmission dioptric system 120,however, are not decentered with respect to one another.

The lenses 123 and 126, which are doublets, each form a lens group unit.The lenses 123 and 126 are not decentered with respect to each other atthe level of a group unit.

As shown in FIG. 10, the image surface of the intermediate image formedbetween the reflecting mirrors 131 and 132 is tilted and curved withrespect to the principal ray of a light beam emitted from the center ofthe projected image surface. This increases latitude for theintermediate image surface, thus facilitating the designing of theentire optical system. The principal ray emitted from the center of theprojected image surface and a principal ray emitted from the margin ofthe projected image surface are substantially parallel to each other onthe last surface of the transmission dioptric system 120 (the exit-sidesurface of the lens 127). The image projection apparatus of FIG. 9 is ofa front projector type, but is convertible to a rear projection type byproviding a reflecting mirror that folds an optical path in the imagingoptical path.

The projection optical system according to this embodiment can project alarge screen without distortion, and the image projection apparatusaccording to this embodiment can be formed with a low-profileconfiguration.

A description is given below of specific embodiments.

In optical system data, surfaces are counted successively from an objectsurface (a surface on which an object to be projected is displayed) as0, 1, 2, . . . with the object surface being 0. Through the followingembodiments, the first and second surfaces are the liquid crystalpanel-side surface and the projection optical system-side surface,respectively, of a polarization beam splitter.

Embodiment 1

Embodiment 1 is a specific embodiment of the image projection apparatusand the projection optical system of FIGS. 7 and 8. That is, Embodiment1 includes the first optical system 71 having positive power andincluding at least one dioptric system and the second optical system 72including at least one reflecting surface having power and havingpositive power as a whole. The first and second optical systems 71 and72 are arranged in the order described from upstream to downstream onthe downstream side of the object surface. An object image istemporarily formed as an intermediate image, and thereafter, is formedas a normal image. The optical elements 712 through 716, 721, and 722are shifted or tilted with respect to the optical axis of the opticalelement having refractive power, which is positioned at the furthestupstream end of the first optical system 71.

The magnification of the intermediate image is approximately 3×.

The data of Embodiment 1 is shown in Table 1.

TABLE 1 Surface Radius of Surface Refractive No. Curvature SeparationIndex Dispersion Shift Tilt Figure 0 ∞ 5.0 1 ∞ 34.30 1.516798 64.1983 2∞ 5.00 3 −2206.4 4.40 1.696802 55.4597 −14.41 −6.3 Spherical 4 −45.82.30 Spherical 5 33.4 8.05 1.696802 55.4597 −8.48 20.8 Spherical 6 69.615.54 Spherical 7 38.6 8.50 1.487489 70.4412 −3.15 −20.8 Spherical 8−54.1 0.55 1.846663 23.7848 Spherical 9 29.0 1.33 Spherical 10  ∞ 35.74Diaphragm 11  130.8 7.90 1.834001 37.3451 2.29 Spherical 12  −72.5 2.00Spherical 13  56.0 14.70 1.743299 49.2216 4.51 Spherical 14  265.1 8.10Spherical 15  −69.5 2.50 1.487489 70.4412 −4.65 Spherical 16  44.2 80.00Spherical 17  3000.0 −150.00 −36.50 −45 Spherical 18  10.00 25Polynomial Free-Form Surface

In Table 1, “Shift” means the amount of shift, and “Tilt” means theamount of tilt. The units of “Radius of Curvature,” “SurfaceSeparation,” and “Shift” are “mm,” and the unit of “Tilt” is “degree.”The same applies to the following embodiments.

The reflecting surface of the second reflecting mirror 722, which is the18^(th)-surface, is an anamorphic polynomial free-form surface havingdifferent vertical and lateral powers. The figure of the anamorphicpolynomial free-form surface having different vertical lateral powers isexpressed by:

Z=X2·x ² +Y2·y ² +X2Y·x ² y+Y3·y ³ +X4·x ⁴ +X2Y2·x ² y ² +Y4·y ⁴ +X4Y·x⁴ y+X3Y2x ³ y ² +Y5y ⁵ +X6·x ⁶ +X4Y2·x ⁴ y ² +X2Y4·x ² y ⁴ +Y6·y ⁶+

where X2, Y2, X2Y, Y3, X2Y2, etc. are coefficients, letting the verticaldirections be the Y directions, the lateral directions be the Xdirections, and the depth of the curved surface be the Z directions. Thevertical (upward and downward) directions and the lateral (rightward andleftward) directions are considered based on the projected image. Thecoefficients of the polynomial free-form surface are shown in Table 2.

TABLE 2 Coefficient Coefficient Value X2 6.99519E−03 Y2 5.16104E−03 X2Y6.00372E−05 Y3 4.11672E−05 X4 −1.12766E−07 X2Y2 4.40932E−07 Y43.71145E−07 X4Y −3.32774E−09 X3Y2 1.09835E−09 Y5 2.32182E−09 X61.30492E−11 X4Y2 −4.80572E−11 X2Y4 −1.76822E−11 Y6 1.14641E−11

In Table 2, “1.14641E-11,” for instance, means “1.14641×10⁻¹¹.” The sameapplies to the following embodiments.

As described above, the first optical system 71 is composed of sevenlenses, and the second optical system 72 is formed of two reflectingsurfaces. The reflecting surface 721 is spherical, and the reflectingsurface 722 is a polynomial free-form surface.

The image surface (screen) of the normal image is a plane surfaceparallel to the rightward and leftward directions of FIG. 7. There is agreat difference in angle of incidence to the screen between a lowerposition (closer to the object) and a higher position (remoter from theobject) of the image height. Therefore, the projected image tends to benarrowed downward and distorted. In this embodiment, distortion on thefinal image surface is corrected by inversely setting the distortion ofthe intermediate image.

FIG. 11 shows the state of image distortion on the final image surface.FIG. 11 shows the state of image distortion when an image displayed on aliquid crystal panel of approximately 0.9 inch diagonal in size ismagnified to approximately 60 inches and projected. As shown in FIG. 11,grid images can be formed at approximately equal intervals, andtrapezoidal distortion may be suitably corrected.

Projection size is 1200×900 mm, magnification is 65× or higher, anddistortion is 0.5% or lower, which values are desirable.

Embodiment 2

Embodiment 2 is a specific embodiment of an image projection apparatusand a projection optical system shown in FIG. 12. In FIG. 12, theprojection optical system of the image projection apparatus is shownenlarged.

The projection optical system includes a first optical system 81 and asecond optical system 82. The first optical system 81 is composed of sixlenses 811 through 816. The second optical system 82 is composed of tworeflecting surfaces 821 and 822. A diaphragm (not graphicallyrepresented) is provided between the lenses 813 and 814.

Like in Embodiment 1, an intermediate image is formed as an invertedimage by the first optical system 81 between the reflecting surfaces 821and 822. The reflecting surface 821, which has positive power andreflects a light beam made incident on the second optical system 82first, has a rotationally symmetric aspheric figure. The reflectingsurface 822 is a polynomial free-form surface. In Embodiment 2, theemployment of a rotationally symmetric aspheric figure makes it possibleto design with higher latitude.

The data of Embodiment 2 is shown in Table 3.

TABLE 3 Surface Radius of Surface Refractive No. Curvature SeparationIndex Dispersion Shift Tilt Figure 0 ∞ 10.00 1 ∞ 30.00 1.516798 64.19832 ∞ 10.00 1 3 −108.87 8.07 1.696802 55.4597 −22.35 3.3 Spherical 4−42.82 17.13 1 Spherical 5 46.85 11.00 1.696802 55.4597 −12.67 9.3Spherical 6 83.16 29.98 1 Spherical 7 −157.78 0.85 1.5168 64.1673 −2.20−38.9 Spherical 8 44.03 1.44 1 Spherical 9 ∞ 13.72 1 Diaphragm 10 103.77 8.23 1.846663 37.3451 −10.17 Spherical 11  −109.21 80.56 1Spherical 12  53.50 6.62 1.7433 37.3451 −5.03 Spherical 13  186.49 16.421 Spherical 14  −138.57 11.00 1.487489 70.4412 1.51 Spherical 15  51.1882.19 1 Spherical 16  1000.00 −150.00 1 −64.67 −45.0 Aspheric 17  10.025.0 Polynomial Free-Form Surface

The figure of the rotationally symmetric aspheric surface employed asthe 16^(th) surface is specified by giving k, A, B, and C in thefollowing well known aspheric equation:

Z=c·r ²/[1+√{1−(1+k)c ² r ² }]+Ar ⁴ +Br ⁶ +Cr ⁸

where Z is an axial depth, c is a paraxial radius of curvature, r is thedistance from an optical axis in a direction perpendicular thereto, k isa conic constant, and A, B, and C are higher-order asphericcoefficients. The same applies to the following embodiments.

The aspheric coefficients of the 16^(th) surface are given in Table 4.

TABLE 4 Conic Constant: k 90.301 4^(th)-order Coefficient: A 4.12759E−086^(th)-order Coefficient: B −5.10327E−12 8^(th)-order Coefficient: C4.43120E−16

The coefficient values of the 17^(th) surface, which is a polynomialfree-form surface, are given in Table 5.

TABLE 5 Coefficient Coefficient Value X2 6.11879E−03 Y2 4.61411E−03 X2Y4.16197E−05 Y3 2.53381E−05 X4 −3.53627E−08 X2Y2 2.62702E−07 Y41.81518E−07 X4Y −9.91605E−10 X3Y2 −3.16955E−11 Y5 1.39821E−09 X63.21795E−12 X4Y2 −5.09377E−12 X2Y4 −5.59615E−12 Y6 7.45481E−12

Embodiment 3

Embodiment 3 is a specific embodiment of an image projection apparatusand a projection optical system shown in FIG. 13.

The projection optical system includes a first optical system 91 and asecond optical system 92. The first optical system 91 is composed offive lenses 911 through 915. The second optical system 92 is formed oftwo reflecting surfaces 921 and 922. The lens 913 is a doublet. Adiaphragm (not graphically represented) is provided between the lenses913 and 914.

Like in Embodiments 1 and 2, an intermediate image is formed between thereflecting surfaces 921 and 922. The intermediate image is formed as aninverted image by the first optical system 91. The reflecting surface921, which has positive power and reflects a light beam made incident onthe second optical system 92 first, has a spherical figure. Thereflecting surface 922 is a polynomial free-form surface.

The data of Embodiment 3 is shown in Table 6.

TABLE 6 Surface Radius of Surface Refractive No. Curvature SeparationIndex Dispersion Shift Tilt Figure 0 ∞ 10.00 6.5 1 ∞ 34.30 1.51679864.1983 2 ∞ 10.00 1 0 3 −41.55 3.85 1.696802 55.4597 −9.32 2.2 Spherical4 −30.93 1.50 1 0 Spherical 5 52.03 6.65 1.696802 55.4597 −10.85Spherical 6 −4072.62 15.54 1 0 Spherical 7 19.64 9.07 1.487489 70.4412−2.17 Spherical 8 174.75 3.86 1.846663 23.7848 Spherical 9 17.83 7.50 10 Spherical 10  ∞ 2.74 1 0 Diaphragm 11  36.07 3.07 1.834001 37.3451−1.32 Spherical 12  −177.66 8.26 1 0 Spherical 13  −33.64 0.65 1.48748970.4412 0.72 Spherical 14  −111.32 50.00 1 0 Spherical 15  3000.00−140.00 1 0 −46.57 −45.0 Spherical 16  1 0 −64.67 −45.0 PolynomialFree-Form Surface

As is apparent from Table 6, the first surface of the lens 911 (the3^(rd) surface in Table 6) is tilted 2.2 degrees. Meanwhile, the lenses912 through 915 are not tilted but shifted with respect to the opticalaxis of the lens 911. The lens 913 of the dioptric system is a doublet,which acts as a group.

The coefficient values of the 16^(th) surface, which is a polynomialfree-form surface, are shown in Table 7.

TABLE 7 Coefficient Coefficient Value X2 7.45075E+11 Y2 5.93127E+11 X2Y6.02215E+07 Y3 4.60347E+07 X4 8.71082E+04 X2Y2 4.41306E+05 Y44.54450E+05 X4Y 3.01303E+03 X3Y2 1.30460E+03 Y5 3.27666E+03 X61.09946E+01 X4Y2 4.38811E+01 X2Y4 1.35219E+01 Y6 1.53991E+00

Embodiment 4

Embodiment 4 has the same optical configuration as Embodiment 3 (FIG.13), but has different data.

The data of Embodiment 4 is shown in Table 8.

TABLE 8 Surface Radius of Surface Refractive No. Curvature SeparationIndex Dispersion Shift Tilt Figure 0 ∞ 10.00 6.5 1 ∞ 34.30 1.51679864.1983 2 ∞ 10.00 1 0 3 −43.18 2.85 1.696802 55.4597 −13.74 0.6Spherical 4 −29.34 1.50 1 0 Spherical 5 61.46 7.25 1.696802 55.4597−10.00 Spherical 6 −368.49 15.54 1 0 Spherical 7 20.42 8.27 1.48748970.4412 −1.71 Spherical 8 158.23 3.75 1.846663 23.7848 Spherical 9 18.544.76 1 0 Spherical 10  ∞ 10.32 1 0 5.00 Diaphragm 11  45.42 2.981.834001 37.3451 −1.26 Spherical 12  −120.79 7.52 1 0 Spherical 13 −91.19 0.55 1.487489 70.4412 −4.68 Spherical 14  118.18 50.00 1 0Spherical 15  3000.00 −140.00 1 0 −38.18 −45.0 Spherical 16  1 0 −64.67−45.0 Polynomial Free-Form Surface

The coefficient values of the 16^(th) surface, which is a polynomialfree-form surface, are shown in Table 9.

TABLE 9 Coefficient Coefficient Value X2 7.77494E−03 Y2 6.11413E−03 X2Y6.97088E−05 Y3 5.28322E−05 X4 −1.09108E−07 X2Y2 5.14945E−07 Y45.13271E−07 X4Y −4.03993E−09 X3Y2 2.45390E−09 Y5 3.04301E−09 X69.74181E−12 X4Y2 −7.12811E−11 X2Y4 −2.11334E−11 Y6 1.03287E−11

Embodiment 5

Embodiment 5 has the same optical configuration as Embodiment 3 (FIG.13), but has different data.

The data of Embodiment 5 is shown in Table 10.

TABLE 10 Surface Radius of Surface Refractive No. Curvature SeparationIndex Dispersion Shift Tilt Figure 0 ∞ 10.00 1 ∞ 34.30 1.516798 64.19832 ∞ 10.00 1 0 3 −33.48 0.80 1.696802 55.4597 −9.32 −4.2 Spherical 4−28.97 1.50 1 0 Spherical 5 116.57 8.85 1.696802 55.4597 −10.85Spherical 6 −61.93 15.54 1 0 Spherical 7 81.87 9.38 1.487489 70.4412−2.17 Spherical 8 −36.19 2.11 1.846663 23.7848 Spherical 9 −63.01 14.391 0 Spherical 10  ∞ 10.20 1 0 Diaphragm 11  19.82 7.50 1.834001 37.3451−1.32 Spherical 12  16.21 21.23 1 0 Spherical 13  −11.77 7.37 1.48748970.4412 0.72 Spherical 14  −15.25 50.00 1 0 Spherical 15  5000.00−140.00 1 0 −44.10 −45.0 Spherical 16  700.00 1 0 10.00 27.0 PolynomialFree-Form Surface

The coefficient values of the 16^(th) surface, which is a polynomialfree-form surface, are shown in Table 11.

TABLE 11 Coefficient Coefficient Value X2 6.54399E−03 Y2 5.58060E−03 X2Y5.34322E−05 Y3 4.38966E−05 X4 −6.71655E−08 X2Y2 3.76878E−07 Y42.54814E−07 X4Y −1.63976E−09 X3Y2 1.70448E−09 Y5 −7.72777E−11 X61.01693E−11 X4Y2 −3.04830E−11 X2Y4 −1.70416E−11 Y6 −4.26270E−13

As described above, each of Embodiments 1 through 5 includes apositive-power first optical system including at least one dioptricsystem and a second optical system having positive power as a whole, thesecond optical system including at least one reflecting surface havingpower. The first and second optical systems are arranged in the orderdescribed from upstream to downstream on the downstream side of anobject. An object image is temporarily formed as an intermediate image,and thereafter, is formed as a normal image. With respect to the opticalaxis of an optical element that is positioned furthest on the objectside in the first optical system and has refractive power, one or moreof the other optical elements are shifted or tilted. In Embodiments 3through 5, with respect to the optical axis of the optical element(lens) 911, positioned furthest on the object side in the first opticalsystem and having refractive power, the other optical elements 912through 915 of the first optical system 91 are not tilted.

In Embodiments 3 and 4, the first optical system 91 is composed of twoor more groups. Of the two or more groups, the lens 913 forming a groupas a doublet is shifted.

In each of Embodiments 1 through 5, at least one of the reflectingsurfaces included in the second optical system is a free-form surface.Of the reflecting surfaces included in the second optical system, thereflecting surface positioned furthest on the side of the position atwhich the normal image is formed is a free-form surface. Further, inEmbodiments 1 through 5, the reflecting surface having positive powerand reflecting a light beam made incident on the second optical systemfirst is rotationally symmetric. In Embodiments 1 and 3 through 5, therotationally symmetric reflecting surface is a spherical reflectingsurface.

In each of Embodiments 1 through 5, the first optical system is formedof only a dioptric system, and the dioptric system of the first opticalsystem excludes an aspheric surface figure.

Accordingly, an image projection apparatus is realized by combining anobject with the projection optical system of any of Embodiments 1through 5.

Embodiment 6

Embodiment 6 is a specific embodiment of the projection optical systemand the image projection apparatus described with reference to FIGS. 9and 10.

The data of Embodiment 6 is shown in Table 12.

TABLE 12 Surface Radius of Surface Refractive No. Curvature SeparationIndex Dispersion Figure Shift Tilt  0 ∞ 22 1 Plane  1 ∞ 34.3 1.62 59.51Plane  2 ∞ 7.1 1 Plane  3 −94 5.06 1.74 44.9 Spherical 8.22 0.264  4−42.9 0.1 1 Spherical  5 140.1 5.02 1.53 66.01 Spherical  6 −106.3 1.951 Spherical  7 54.2 15.69 1.5 69.24 Aspheric  8 −39.6 17.47 1.76 27.59Spherical  9 41.2 21.71 1 Spherical 10 −79.3 5.74 1.75 34.1 Spherical 11−33 1.35 1 Spherical 12 ∞ 93.3 1 Plane Diaphragm 13 −50.6 25 1.63 57.93Spherical 14 −159.1 42.86 1 Spherical 15 186.7 25 1.68 31.56 Aspheric 1696.1 47.89 1.56 63.6 Spherical 17 −553.4 50 1 Spherical 18 212.4 25 1.7235.45 Spherical 19 157.6 185 1 Polynomial Free-Form Surface 20 10000−266 1 Axially 13.68 46 Symmetric Reflecting Surface 21 0 780.85 1Polynomial 77.69 −32.9 Free-Form Surface

The aspheric coefficients of the 7^(th) and 15^(th) surfaces are givenin Table 13.

TABLE 13 Coefficient 7^(th)Surface 15^(th)Surface K 0 0.286791 A−1.35E−06 −6.00E−09 B −1.56E−09 −1.22E−11 C 2.72E−13 8.92E−16 D−4.35E−15 1.40E−20 E 0.00E+00 4.55E−23 F 0.00E+00 −1.29E−26 G 0.00E+007.10E−31 H 0.00E+00 1.73E−34 I 0.00E+00 −1.83E−38

The coefficient values of the 19^(th) and 21^(st) surfaces, which arepolynomial free-form surfaces, are given in Table 14.

TABLE 14 Coefficient 19^(th)Surface 21^(st) Surface X2 −0.00095 0.002876Y2 −0.00096 0.00173 X2Y −1.94E−07 −1.03E−05 Y3 −3.80E−07 −5.02E−06 X4−1.06E−07 3.07E−09 X2Y2 −2.11E−07 5.56E−08 Y4 −1.01E−07 2.42E−08 X4Y5.54E−11 1.17E−11 Y5 −4.67E−11 −1.45E−10 X6 −7.62E−14 −1.72E−15 X4Y2−8.23E−13 −7.17E−13 X2Y4 −1.19E−16 1.19E−13 Y6 1.20E−13 6.52E−13

MTF performance and distortion on a screen by the projection opticalsystem of Embodiment 6 are 60% or higher and 2% or lower, respectively,at a frequency of 0.5 c/mm.

In Embodiment 6, the screen onto which a normal image is projected is 60inches in size. The maximum width of the projection optical system in adirection perpendicular to the screen is 472 mm.

MTF performance at an evaluation frequency of 0.5 c/mm was examined,setting grid lines of ±1.0Y, ±0.5Y, 0.0Y, ±1.0X, ±0.5X, and 0.0X alongthe X axis (rightward and leftward directions) and the Y axis (upwardand downward directions) on the screen as shown in FIG. 14. Table 15shows the results (MTF values) of the examination.

TABLE 15 0.0 X 0.5 X 1.0 X   1.0 Y 76.7% 74.3% 73.4%   0.5 Y 71.5% 74.7%75.4%   0.0 Y 83.1% 82.1% 79.7% −0.5 Y 91.2% 85.0% 83.9% −0.1 Y 92.6%71.6% 83.1%

FIG. 15 shows MTF characteristics in the saggital direction (s) and themeridional direction (m) at ±1.0Y and 0.0Y in the range of frequenciesof 0 to 0.5 c/mm at X=0.0X. FIG. 16 shows MTF characteristics in thesaggital direction (s) and the meridional direction (m) at ±1.0Y and0.0Y in the range of frequencies of 0 to 0.5 c/mm at X=0.5X. FIG. 17shows MTF characteristics in the saggital direction (s) and themeridional direction (m) at ±1.0Y and 0.0Y in the range of frequenciesof 0 to 0.5 c/mm at X=1.0X. FIGS. 15 through 17 show that Embodiment 6has good MTF characteristics.

According to the projection optical system of Embodiment 6, a reflectiondioptric system includes first and second reflecting mirrors arranged inthe order described from upstream to downstream on the downstream sideof the transmission dioptric system, and the intermediate image surfaceof a projected object surface is positioned between the first and secondreflecting mirrors. The first reflecting mirror has a negative-power,axially symmetric reflecting surface (the 22^(nd) surface). The secondreflecting mirror has an anamorphic polynomial free-form surface havingdifferent vertical and lateral powers (the 23^(rd) surface). Ananamorphic polynomial free-form surface having different vertical andlateral powers (the 19^(th) surface) is provided in the transmissiondioptric system as a part correcting the aspect ratio of theintermediate image surface of a projected object surface.

It is possible to correct the aspect ratio of the intermediate imagethrough the figure of a reflecting mirror in the reflection dioptricsystem. However, it is desirable that the figure of the reflectingmirror of the reflection dioptric system be determined mainly in termsof distortion correction. Accordingly, it is desirable that the aspectratio be adjustable beforehand in the transmission dioptric system.Accordingly, it is effective to employ the above-described anamorphicpolynomial free-form surface as a part correcting the aspect ratio inthe transmission dioptric system.

The number of polynomial free-form surfaces employed in the transmissiondioptric system is not limited to one. However, according to Embodiment6, the employment of only the single polynomial free-form surface (the19^(th) surface) in the transmission dioptric system produced sufficientcorrection effect. The polynomial free-form surface employed in thetransmission dioptric system may be positioned close to the projectedobject surface. However, it is desirable that the polynomial free-formsurface be positioned close to the projection surface side in thetransmission dioptric system in order to increase the correction effect.

In the transmission dioptric system, an NA (=0.143) on the projectedobject surface side is greater than an NA (=0.01) on the intermediateimage surface side. The magnification of the intermediate image M1(=1.5) falls within the range of 1 to 5. The magnification of projection(=75×) is 40× or higher. The angle of projection to a projection surfaceθ (=11 degrees) is 5 degrees or greater.

In the case of forming the transmission dioptric system, its NA on theprojected object surface side (hereinafter, an NA1) is determined by theorientation distribution characteristics of an illumination system,while its NA on the intermediate image surface side (hereinafter, anNA2) is changeable by the arrangement and the configuration of thetransmission dioptric system. In order to increase magnification ofprojection, it is effective to increase the power of the reflectiondioptric system. This, however, reduces the focal length of thereflection dioptric system on its image or downstream side so that thefocal point of light beams is shifted to the reflecting mirror side ofthe reflection dioptric system. As a result, only a small-size normalimage can be formed. That is, magnification is reduced. In order toeliminate this disadvantage, the NA2 of light beams incident on thereflection dioptric system was focused on. As a result, it wasdetermined that making the NA2 smaller than the NA1 had a remarkableeffect in increasing the projection optical system magnification.

It was determined that the NA2 might be 0.005 to 0.01 to realize amagnification projection optical system magnifying and projecting theimage of a projected object surface of 0.9 inch diagonal size onto a60-inch screen with a thickness of 500 mm or less. If the NA2 is overlyreduced, the overall length of the transmission dioptric systemincreases. Accordingly, the NA2 is desirably 0.005 to 0.01, consideringthe downsizing of the entire apparatus.

If the NA2 is set to 0.01 or greater, the transmission dioptric systemis made compact. However, an increase in the NA tends to make itdifficult to perform distortion correction on the projection screen orensure magnification performance. The upper limit of the NA2 may be 0.01or greater for a screen size smaller than 60 inches.

The image projection apparatus of the present invention may be of afront projector type or of a rear projection type with a folding mirrorfolding back an imaging optical path.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

The present application is based on Japanese priority patentapplications No. 2003-029595, filed on Feb. 6, 2003, No. 2003-029602,filed on Feb. 6, 2003, and No. 2003-409304, filed on Dec. 8, 2003, theentire contents of which are hereby incorporated by reference.

1. A projection optical system for use in an image projection apparatusprojecting an image of an object and forming a final image on a plane,the projection optical system comprising: first and second opticalsystems arranged so that the first optical system is closer to theobject than is the second optical system; wherein the first opticalsystem includes at least one dioptric system and has positive power, thesecond optical system includes at least one reflecting surface havingpositive power, the image of the object is formed as an intermediateimage in an optical path of the first and second optical systems, andthe intermediate image is magnified and projected, and the secondoptical system is configured so that an angle of incidence to aprojection surface on the plane is greater at a position farthest fromthe object than at a position closest to the object on the final image.2. The projection optical system as claimed in claim 1, wherein adifference between the angle of incidence at the position of the finalimage farthest from the object and the angle of incidence at theposition of the final image closest to the object is 53 degrees or more.3. The projection optical system as claimed in claim 1, wherein thefirst optical system is configured to distort the intermediate image soas to offset a possible trapezoidal distortion of the final image. 4.The projection optical system as claimed in claim 1, wherein the firstoptical system is configured to form the intermediate image with adistortion inverse to a possible trapezoidal distortion of the finalimage.
 5. The projection optical system as claimed in claim 1, whereinthe first optical system is configured to form the intermediate imagedistorted inversely relative to the final image narrowed downward anddistorted.
 6. The projection optical system as claimed in claim 2,wherein the first optical system is configured to distort theintermediate image so as to offset a possible trapezoidal distortion ofthe final image.
 7. The projection optical system as claimed in claim 2,wherein the first optical system is configured to form the intermediateimage with a distortion inverse to a possible trapezoidal distortion ofthe final image.
 8. The projection optical system as claimed in claim 2,wherein the first optical system is configured to form the intermediateimage distorted inversely relative to the final image narrowed downwardand distorted.
 9. The projection optical system as claimed in claim 1,wherein the first optical system is configured to tilt the intermediateimage with respect to a principal ray of a light beam emitted from acenter of a surface of the object surface.
 10. A projection opticalsystem for use in an image projection apparatus projecting an image ofan object and forming a final image on a plane, the projection opticalsystem comprising: first and second optical systems arranged so that thefirst optical system is closer to the object than the second opticalsystem; wherein the first optical system includes at least one dioptricsystem and has positive power, the second optical system includes atleast one reflecting surface having positive power, the image of theobject is formed as an intermediate image in an optical path of thefirst and second optical systems, and the intermediate image ismagnified and projected, and the second optical system is configured sothat an optical path from the reflecting surface having the positivepower to the final image is longer at a position thereof farthest fromthe object than at a position thereof closest to the object.
 11. Theprojection optical system as claimed in claim 10, wherein a differencebetween the optical path from the reflecting surface having the positivepower to the position of the final image farthest from the object andthe optical path from the reflecting surface having the positive powerto the position of the final image closest to the object is 663 mm ormore.
 12. The projection optical system as claimed in claim 10, whereinthe first optical system is configured to distort the intermediate imageso as to offset a possible trapezoidal distortion of the final image.13. The projection optical system as claimed 10, wherein the firstoptical system is configured to form the intermediate image with adistortion inverse to a possible trapezoidal distortion of the finalimage.
 14. The projection optical system as claimed in claim 10, whereinthe first optical system is configured to form the intermediate imagedistorted inversely relative to the final image narrowed downward anddistorted.
 15. The projection optical system as claimed in claim 11,wherein the first optical system is configured to distort theintermediate image so as to offset a possible trapezoidal distortion ofthe final image.
 16. The projection optical system as claimed in claim11, wherein the first optical system is configured to form theintermediate image with a distortion inverse to a possible trapezoidaldistortion of the final image.
 17. The projection optical system asclaimed in claim 11, wherein the first optical system is configured toform the intermediate image distorted inversely relative to the finalimage narrowed downward and distorted.
 18. A projection optical systemfor use in an image projection apparatus projecting an image of anobject and forming a final image on a plane, the projection opticalsystem comprising: first and second optical systems arranged so that thefirst optical system is closer to the object than the second opticalsystem; wherein the first optical system includes at least one dioptricsystem and has positive power, the second optical system includes atleast one reflecting surface having positive power, the image of theobject is formed as an intermediate image in an optical path of thefirst and second optical systems, and the intermediate image ismagnified and projected, and the first optical system is configured toform the intermediate image with a distortion inverse to a possibletrapezoidal distortion of the final image.
 19. The projection opticalsystem as claimed in claim 18, wherein the first optical system isconfigured to distort the intermediate image so as to offset thepossible trapezoidal distortion of the final image.
 20. The projectionoptical system as claimed in claim 18, wherein the first optical systemis configured to form the intermediate image distorted inverselyrelative to the final image narrowed downward and distorted.